The ATLAS experiment at CERN announces the strongest evidence so far that the Higgs gives mass to matter particles – University of Copenhagen

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28 November 2013

The ATLAS experiment at CERN announces the strongest evidence so far that the Higgs gives mass to matter particles

The Discovery tau ATLAS team has been involved in laying the grounds for a new result from ATLAS, namely the announcement of very strong evidence for the Higgs boson decaying to tau leptons.

In order to prove that the Higgs field is responsible for the generation of the masses of the matter particles, we have to observe the decay of the Higgs boson also to these very particles, and this is exactly why the new results are so important. The Discovery team at the Niels Bohr Institute has been involved from the very beginning of ATLAS, and have contributed essentially to the identification of tau lepton decays both at the data collection level and more directly for the analysis, and have coordinated these activities for the past three years.

In the Standard Model of particle physics, the fundamental building blocks of nature are quarks (which live inside hadrons) and leptons (such as the electron, and its heavier siblings, the muon and the tau). These building blocks interact with each other via fundamental forces carried by bosons - the photon carries electromagnetism, the W and Z bosons carry the weak nuclear force, and the gluon carries the strong force.

In the Standard Model, all these particles (except the photon and the gluon, which are massless) acquire their mass by interacting with the Higgs field.

A collision event in the CERN LHC, as measured by the ATLAS detector, looking very much like a Higgs boson decaying to a pair of tau leptons

The Higgs boson, discovered in July 2012 through its decay into pairs of photons and of Z and W bosons, is a clear manifestation of the existence of the Higgs field and a proof that we understand how the force carrying bosons acquire mass. However, to prove that the Higgs field is also responsible for the generation of the masses of the matter particles, we have to observe the decay of the Higgs boson also to these very particles.

When produced, a Higgs boson decays very quickly, and there are several options open to it in terms of what particles it can produce when it decays. If a particle gets its mass from the Higgs boson, then the Higgs boson will sometimes decay to that particle. And the heavier the particle, the more often it will show up in Higgs boson decays. Therefore much attention has been paid to the search for the Higgs boson decay into pairs of the heaviest of the leptons – the tau lepton, which has a mass about twice of that of the proton.

Mass distribution of tau pairs reconstructed in the ATLAS detector. The fact that the red line (which includes a Higgs decaying to taus) agrees better with the data (black spot) than the other lines is evidence that the Higgs does, indeed, give mass to leptons.

What ATLAS has now announced is very strong evidence for the Higgs boson decaying to tau leptons. Tau leptons themselves decay pretty quickly, and produce at least one invisible neutrino when they do so. Hence, spotting Higgs decays to pairs of tau leptons is difficult and reconstructing the mass of the decaying Higgs boson is hard. The evidence is the result of a complex analysis which makes use of the fact the we now know the mass of the Higgs to be about 130 times the proton mass.

The analysis has taken more than one year to carry through for a sizeable group of ATLAS physicists. Until one week ago the data was still blinded, and the entire analysis was designed and scrutinized without knowledge of the final result. When the data was finally unblinded last week it confirmed the prediction of the Standard Model: The fraction of Higgs bosons decaying to tau leptons is consistent with expectations.